In 1997, the U.S. Environmental Protection Agency (EPA) estimated that 10 percent of the nation's lakes, rivers, and bays have sediment contaminated with toxic chemicals that can kill fish living in those waters or impair the health of people and wildlife who consume contaminated fish or water. The magnitude of the sediment contamination problem in the United States is evidenced by the more than 2,100 state advisories that have been issued against consuming fish. An important class of such contaminants are hydrophobic organic compounds (HOCs), which includes polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), pesticides (such as DDT), and some organometallic compounds (such as dimethyl mercury). HOCs are important contaminants of concern in sediments because of their association with fine-grained, organic-rich sediment material. HOCs persist in sediments for many years and exhibit the potential for bioaccumulation and toxicity. Thus, HOCs in sediments pose risks to human health and the environment.
The importance of the HOC contamination of sediments is exemplified by PCBs, which are long-lived in sediments and leach into overlying water, accumulate in benthic invertebrates, and transfer through the food chain to animals and humans. This has resulted in 697 fish consumption advisories for PCBs in the U.S. in 1998.
The cost of remediating contaminated sediments often runs into billions of dollars. The most commonly considered technologies for contaminated sediment management are (1) dredging and placement in confined disposal facilities (CDFs) or hazardous waste landfills and (2) capping, an option for containment in engineered subaqueous sites. However, either option is expensive and requires large-scale material handling and long-term management. Further, since most PCB-contaminated sediment sites are large, the application of any remediation option is a difficult task.
Traditional sediment treatments based on dredging are costly and problem-prone and will leave residual PCBs, and thus do not completely eliminate environmental and human health risks. In addition, dredging operations can cause temporary high levels of contaminants in the water column due to resuspension of buried sediments and release of pore water. Dredging also destroys aquatic habitats. Further, all known sediment treatment technologies will leave residual contaminants. In situ treatment is thus needed even if dredging is employed.
Therefore, active, in situ stabilization methods that do not involve sediment relocation are attractive.
Prior in situ sediment stabilization efforts include capping with clean sand and geofabric. Some major unresolved issues regarding the long-term efficiency of caps include maintenance of cap integrity during high flow or storm events, physical alteration of the indigenous habitat, and organisms that can burrow into the caps and impair their efficacy.
U.S. Pat. No. 5,391,300, “METHOD FOR THE REMOVAL OF HALOGENATED ORGANIC COMPOUNDS FROM AN ENVIRONMENT”, issued to Webb et al. and assigned to General Electric Company of N.Y., U.S.A., Feb. 21, 1995, discloses sorbing halogenated organic compounds such as PCBs and PAHs from aqueous solutions and mixtures by a variety of organic polymeric materials. Recently, the use of Ambersorb polymeric resin has been explored for in situ remediation and containment of contaminants, as reported by West, W. W.; Kosian, P. A.; Mount, D. R.; Makynen, E. A.; Pasha, M. S.; Sibley, P. K.; Ankley, G. T. “Amendment of Sediments with a Carbonaceous Resin Reduces Bioavailability of Polycyclic Aromatic Hydrocarbons”, Environmental Toxicology and Chemistry, 20, 1104–1111. The major uncertainties regarding the feasibility of these approaches are: the high cost of the sorbent material, the permanence of the treatment effectiveness, and the lack of knowledge of sequestration processes controlling phase transfer and stabilization of the contaminants.
There exists a continuing need, therefore, for an inexpensive, nonremoval, in situ remediation technology for submerged sediments contaminated with persistent hydrophobic organic compounds.